Linear polyesters, such as poly(ethylene terephthalate) or PET, and poly(butylene terephthalate) or PBT, are widely used in the preparation of articles by forming methods such as injection molding and tube extrusion. Many of their properties, including chemical stability, solvent resistance and low permeability to gases, make them attractive candidates for such forming operations as blow molding, profile extrusion and thermoforming. One problem in such operations is the relatively low melt viscosities of the polyesters, as a result of which the formed articles do not adequately retain their shape immediately after forming and before they have cooled.
In recent years, various methods have been developed for increasing the melt viscosities and melt strengths of such polyesters.
Kodama et al., U.S. Pat. No. 4,141,882, describe obtaining a polyester composition having high melt viscosity by blending a polyester comprising at least 80 mole percent of ethylene terephthalate units with (1) an epoxy compound of isocyanuric acid construction (A) and at least one organic compound (B) capable of reacting with the epoxy compound (A), or (2) a product of the melt reaction of epoxy compound (A) with the organic compound (B).
Blaschke et al., United Kingdom Patent No. 2,098,231 describe shaped bodies formed of polytetramethylene terephthalate stabilized with triglycidyl isocyanurate (TGIC) or a bisoxazoline.
Yosihara, U.S. Pat. No. 4,795,771, describes polyesters exhibiting crystallization at low temperatures and having high heat distortion temperatures and good dimensional stability. The disclosed polyester compositions consist of polyester, of which 80% is poly(ethylene terephthalate), a carboxylic acid ester of a polyalkylene glycol, an epoxy compound having a polyoxyalkylene and an inorganic filler such as talc.
Rawlings, U.S. Pat. No. 533,679, describes reinforced polyesters consisting of a polyester resin, a polyepoxy having more than two epoxide functionalities and a reinforcing agent.
Borman et al., U.S. Pat. No. 4,020,122, describe a method to increase the melt elasticity or viscosity of linear high molecular weight polyesters. The patentees teach adding to the polyester organic polyepoxides having at least two epoxide groups per molecule.
Korver, U.S. Pat. No. 4,071,504, disclose low carboxyl content polyester fibers. The fibers are produced by melt extruding a polyester and a catalytic material, such as an alkali metal salt, and adding and reacting therewith monofunctional epoxides.
Brown et al., U.S. Pat. No. 4,904,746, teach producing branched polyesters having advantageous melt viscosity properties. The improved method comprises forming a reactive concentrate by reacting (A) triglycidyl isocyanurate with (B) a linear polymer having ester and free carboxylic groups. The reactive concentrate is then melt blended with (C) a linear polyester with free carboxylic groups.
McCracken et al., U.S. Pat. No. 4,933,429, teach the production of high melt viscosity branched polyesters. The disclosed branched polyesters are produced by effecting a reaction between (A) a polyester having a substantial portion of free carboxylic groups, (B) a polyepoxy compound, preferably having at least three epoxy functionalities, and (C) a catalyst selected from salts of aliphatic carboxylic acids and primary alkylamines.
Kometani et al., U.S. Pat. No. 4,246,378, further describe a polyester having improved melt stability. The patentees teach preparing a composition having 100 parts by weight of polyester, 0.1-40 parts by weight of an epoxy compound, and 0.001-10 parts by weight of organic sulfonate salts and organic sulfate salts.
Borman et al., U.S. Pat. No. 3,886,104, teaches stabilizing high molecular weight polyester resins by adding to the polyester resin a stabilizer comprising an internally polyfunctional epoxide having at least two epoxide functionalities. The epoxide containing stabilizers disclosed to be useful are epoxidized polyunsaturated triglycerides.
Commonly assigned U.S. patent application Ser. No. 07/526,579, filed May 17, 1990 now abandoned discloses branched polyesters having enhanced melt viscosity. The application describes adding a reinforcing mineral to a polyester resin branched with a TGIC branching agent.
However, the above-described polyester compositions still suffer from processing drawbacks at high melt viscosities. This processability difficulty resulted from melt stiffness encountered when larger amounts of polyepoxide functional agents were employed. Thus, there exists in the art a need for high melt viscosity linear polyester resins which are easily processable.
Surprisingly, it has now been found that compositions comprising linear polyesters, difunctional epoxides and salts of aliphatic carboxylic acids provide a composition which possesses both excellent hydrolytic stability and melt viscosity stability. Further, preferred polyester compositions possess high melt viscosity and ease of processability. The compositions of the present invention unexpectedly exhibit improved results over compositions employing mono- and tri-functional epoxides.
It is further noteworthy that while compositions containing linear polyesters and branched polyesters have good hydrolytic stability, surprisingly the linear polyester compositions of the present invention are better than the branched in this respect.